Few areas in Pediatrics have remained as controversial as the design of Neonatal Intensive Care Nurseries and their impact on development of premature infants. Although health care providers have generated many ideas about the types and patterns of environmental stimulation that may optimize development, there is no scientific consensus on what constitutes an "appropriate" environment. The goal of this proposal is to characterize the circadian system of the developing preterm infant and to explore by way of environmental intervention how the Neonatal Intensive Care Unit (NICU) environment influences long-term physiology, behavior and clinical outcomes. In order to achieve these goals the investigators will examine: (1) the NICU and home ambient lighting and sound level from a chronobiological perspective; and (2) the output of the infant circadian system: heart rate, body temperature and sleep-wake rhythms. They have demonstrated that the major afferent pathway (retinohypothalmic) to the biological clock is present as early as 29 weeks postconceptional age, and that nationwide NICU illumination varies considerably and randomly over time and between bedsides. Since light is the most important zeitgeber of circadian timing system, the preterm infant developing circadian system may be affected by the illumination. Developmental benchmarks have been established in their laboratory that describe the ontogeny of the circadian system in both premature and full term infants, and their newly developed video somnography will allow quantitative measurement of neonatal sleep and circadian rhythms in the hospital and home. The focus of these studies is to use developmental benchmarks to evaluate the effect of environmental intervention, namely 24 hr rhythmic patterns of light, in the NICU on quantitative measures of outcome. Light stimuli have a powerful influence on sleep and/or biological rhythmicity in adults; however, little data are available to assess their effects on hospitalized preterm infants. They hypothesize that light-dark cycling will facilitate sleep consolidation and the development of circadian rhythmicity in preterm infants, and ease the transition from the nursery to the home and that the long-term effects of the photoperiodic stimulation will persist at home. They expect that the preterm infants in the intervention group will grow faster, have less time on ventilator, start nippling earlier, will be neurologically more mature at 36 wk postconceptual age and will be discharged sooner. At 1 and 3 months of age, these infants will show more mature circadian rhythms, show better nighttime settling and will be neurologically and clinically more mature than controls.